(1) Field of the Invention
This invention relates to stents also known as expandable intraluminal grafts for use within a body passageway or duct.
(2) Description of the Related Art
Stents have become the object activity in recent years. Particular reference is made to the patent to PALMAZ, Patent Number 4,733,665, issued on Mar. 29, 1988, and the U.S. Patents, foreign patents and other publications referenced therein.
In the prior art, metals have been the primary material from which stents were made. Some workers in the field, for example, PALMAZ in his patent 4,733,665 has indicated that "any suitable plastic material having the required characteristics previously described" are suitable. Column 6 line 27. Those stents available commercially on the market and about which many research papers have been written, have almost exclusively been of metal.
Trouble has been experienced with some of the stents. It is believed that turbulence of fluid flowing through the stents is one of the problems. According to the stents illustrated in PALMAZ'665, some of them are from woven wires whereas at wire intersections there is a certain amount of irregularity that causes turbulence. Also, when a slotted stent is expanded the bands of metal are twisted in the expansion process so that they project into the lumen somewhat. Also as PALMAZ shows, in both cases the expansion process will reduce the length of the stent.
Obviously the metal stent is quite rigid in comparison to the passageways or ducts. This rigidity in comparison to the more flexible body tissues is in itself an incompatibility which may cause adverse reactions of the body against the foreign material introduced as a stent.
In addition the metals are incompatible in the way enzymes and endothelia react therewith. In particular, metal stents have been associated with the process of neointimal hyperplasia, the excessive overgrowth of intimal endothelial cells that grow to cover the stent's surface. This process of neointimal hyperplasia has been shown to cause restenosis of stents in some cases.
Before this invention it was known that the thermoplastic polymers, above their glass transition temperature, can be elongated or stretched 200 to 500% of their original length before breaking. Elongation increases both the tensile strength and the modulus of the polymer. This phenomena is generally known as necking. This is illustrated in FIGS. 1-A, 1-B, 1-C, and 1-D, wherein a test specimen is subject to increasing stress. First, when a test specimen having a uniform test coupon area, as shown in FIG. 1-A, is elongated there will be an area of yield with a plastic deformation and a corresponding reduction of area. (FIG. 1-B) This area will reduce to a definite amount and then continued stress will not result in a continued reduction of area of the neck but will bring continuing material from the coupon or test area into the neck (FIG. 1-C) until the entire specimen portion is of a reduced area. This is shown in FIG. 1-D. At this point, additional strain will result in a rupture of the neck without significant reduction in cross-sectional area.
This phenomena is described in detail by McCrum, N. G., C. P. Buckley, & C. B. Bucknall, "Principles of Polymer Engineering," pp. 168-172, Oxford University Press, Oxford, (1988).